Update to 3.21

Upstream changes:
version 3.21: Wed 11 Jan 10:02:19 CET 2017

Fixes:
- XOP hooks were instated, even though XML::Compile::XOP was not.
  Schema/WSDL compilation will be a little faster now.
  https://github.com/mjgardner/xml-compilex-schema-loader/issues/1
  [Slaven Rezi�]
- constant name: XS_DAEMON_NS -> XC_DAEMON_NS
  rt.cpan.org#119800 [Pavel Trushkin]

Improvements:
- spell fix Debian rt.cpan.org#118577 [Gregor Herrmann, Debian]

(wen)